1) Field of the Invention
The present invention relates to a voltage stabilizer that stabilizes a voltage of a circuit.
2) Description of the Related Art
Conventionally, a power supply circuit that is connected to a battery has a problem in that after the power supply circuit passes an excessive current from the battery, the voltage of the battery drops rapidly and the output voltage of the power supply circuit becomes unstable. Therefore, a power supply control circuit is used to stabilize the output voltage of the power supply circuit.
The power supply control circuit obtains signals of two systems concerning the current that flows through the power supply circuit and the output voltage of the power supply circuit. Based on the obtained signals of the two systems, the power supply control circuit determines the current and the voltage of the power supply circuit. When an excessive current flows through the power supply circuit, the power supply control circuit controls the current from the battery, thereby stabilizing the voltage of the power supply circuit (see, for example, Japanese Utility Model Application Laid-Open No. H5-25945).
FIG. 11 is an example of a conventional power supply circuit. As shown in FIG. 11, this power supply circuit has a battery 10 that generates electromotive force, a load 20 that applies load to the power supply circuit, and a direct current to direct current (DC-DC) converter 30 that converts a direct current at a voltage of the battery 10 to a direct current at a different voltage.
The DC-DC converter 30 has a capacitors 40a and 40b, a diode 50, a transistor 60, a coil 70, voltage dividing resistors 80 and 90, a current detecting circuit 100, and a power supply control circuit 110.
The voltage dividing resistors 80 and 90 are used to detect an output voltage of the power supply circuit. The current detecting circuit 100 detects a size of a current that flows from the battery 10. The power supply control circuit 110 is connected to the current detecting circuit 100, an intermediate point between the voltage dividing resistors 80 and 90, and the transistor 60. The power supply control circuit 110 obtains a signal concerning a current from the current detecting circuit 100, obtains a signal concerning an output voltage from the intermediate point between the voltage dividing resistors 80 and 90, and controls the transistor 60.
In other words, the power supply control circuit 110 obtains signals of the two systems of the output voltage and the current of the power supply circuit, from the intermediate point between the voltage dividing resistors 80 and 90, and from the current detecting circuit 100. When an excessive current flows from the battery 10, the power supply control circuit 110 controls the transistor 60 to prevent the flow of the excessive current, thereby stabilizing the output voltage of the power supply circuit.
According to the conventional technique, however, circuit parts that are used generally cannot be used to prepare the power supply control circuit of the power supply circuit having an inherent characteristic. It is necessary to develop own circuit parts corresponding to individual power supply circuits, to prepare the power supply control circuit, which requires an enormous amount of cost.
In integrating circuit parts and preparing the power supply control circuit using integrated circuit (IC) chips to respond to the requirement for miniaturization of the power supply control circuit, IC chips that can handle control signals of two or more systems are necessary. However, such IC chips are not available and therefore need to be developed, which is a more important task.
In other words, it is considerably important to stabilize the output voltage of the power supply circuit using circuit parts that are generally used without providing a special power supply control circuit in the power supply circuit.